A standard domestic water heater for home or small commercial use consists of a pressure tank containing water at line pressure (20 to 100 psi) and a heating device. The heating device can be an electric element gas or oil burner or heat exchanger transferring heat from another source.
The pressure tank provides sufficient storage of heated water to satisfy peak demands. The heating device will not transfer heat fast enough to satisfy most demand rates but given sufficient time will bring the water in the tank back to the desired temperature.
The vast majority of pressure tanks are made of steel and protected from corrosion by a glass or cement lining. The corrosion protection is not complete and the life of a typical tank is about 10 years. The tank must withstand the line pressure safety and steel is the lowest cost way of getting the required structure.
Tanks can be made much less expensively if they contain water at ambient (atmospheric) pressure. The present invention allows the line pressure to be transferred from the entering cold water stream to the leaving hot water stream without imposing this pressure on the walls of the tank. The pressure is contained in 2 small vessels with a combined volume that may be much less than one percent the volume of the tank. These vessels are constructed so that one can be filled with the heated water while the other discharges its contents to the hot water line and vice versa. They are cyclically operated by the pressurized entering fluid stream.
By this mechanism the line pressure can be contained in a small device utilizing minimum materials while the heated volume of water is contained in a low cost tank at ambient pressure.